(a) Field of the Invention
The subject invention relates to an airborne survey aircraft or towed geophysical survey bird exploration system and more particularly, but not by way of limitation, to a remote sensing electric field exploration system for exploration of oil and gas, mineral deposits and aquifers and using natural electromagnetic fields as an energy source. This invention is also applicable to offshore exploration using a towed geophysical “fish” system. It is further applicable to the detection of per-curser events ahead of earthquakes and volcanic eruptions.
(b) Discussion of the Prior Art
Heretofore, airborne electromagnetic systems have been in use for natural resource exploration from about 1950 onwards. These systems depend mainly upon the measurement of the magnetic and conductive properties of the underlying ground. Airborne magnetic survey systems, that employ magnetometers with advanced stages of development, provide very satisfactory results. However, airborne conductivity measurements of the underlying terrain made with airborne electromagnetic systems that currently exist, leave a great deal of room for improvement.
Electromagnetic systems typically operate at a minimum terrain clearance with respect to safety and employ electromagnetic transmitters operating in the frequency range from about 20 Hz. to 50 kHz with limited ground penetration. The ground currents and their related secondary magnetic fields as induced in the underlying ground by these transmissions are detected by receiving coils mounted in a tail boom on an aircraft or in an airborne survey bird towed behind the aircraft. Either fixed wing aircraft or helicopters are used for these surveys. The response from the underlying ground is related to it's conductivity and the depth of penetration of the transmitted fields. The latter is primarily a function of the frequency employed and the field strength of the electromagnetic field that is generated by the equipment. Typical maximum penetrations are in a range of 400 to 1000 ft.
The only exception to the above description was an airborne system known as “AFMAG” that was developed by S. H. Ward and others in the 1960's. (S. H. Ward et al. AFMAG-Applications and Limitations. Geophysics, Vol. XXXI, No. 3 (June 1966), pp. 576–605.) This system utilized the natural electromagnetic fields generated by lightning events occurring in distant electrical storms. These storms can provide a source for electromagnetic energizing of the ground, primarily in the frequency range of 20 Hz. to 500 Hz. Useable frequencies down to about 3 Hz. exist but high quality receiving coils and coil anti-vibration mountings are required for the lower frequencies. These were apparently not available in the AFMAG system.
Although the AFMAG system showed some promise, it did not achieve sufficient commercial acceptance to survive for more than a short period. Amongst the various problems of the system was the absence of the sophisticated instrumentation and digital data acquisition and processing systems that were not available at that time. Also and very importantly, there was a lack of adequate technology for suppressing the prime sources of noise, such as angular vibration of the detection coils in the presence of a strong magnetic field in the earth. The latter is associated with a motor generator effect that can detect a millionth of a degree of angular vibration.
The AFMAG system was also restricted to the use of audio frequency fields and did not employ extremely low frequency and much more powerful natural magnetotelluric fields, as used in the present invention. Just as importantly, the AFMAG system as well as all other airborne electromagnetic systems, past or present, did not make use of the valuable data available in the electric field components of electromagnetic fields.
The subject invention demonstrates that electric field data, as measured by methods that do not make contact with the ground, can be more important than the magnetic component of electromagnetic fields. Experience with the invention has also shown that, for specific reasons, the measuring of the electric field data is particularly valuable at frequencies below 3 Hz. This type of information is completely missing in the old AFMAG system as well as current airborne electromagnetic systems. The electric field data employed in the present invention lies in the range of frequencies from 0.1 Hz. to 3 Hz. and is used for the airborne detection of an induced polarization phenomena. Also, the related responses of dielectric interfacial polarization effects can be detected over aquifers and oil and gas fields. These low frequency polarization effects, which are strongly expressed in the electric fields, are discussed herein.
An important factor in comparing the subject airborne method with other airborne systems, is that for certain specific reasons, as presented, the operation can function at unusually high terrain clearances of 1000 ft. to 2000 ft. All other airborne systems that use transmitters have to fly at clearances in the range of 300 to 500 ft., which adds to problems related to interferences from power lines and pipelines. These interferences badly degrade data quality, often in areas where discovery potential is the greatest. With the present invention operating at greater clearances and at low frequencies, these problems disappear for all practical purposes.
In U.S. Pat. No. 6,765,383 issued to the subject inventor, a magnetotelluric geophysical survey system is described using an aircraft survey bird. The survey system uses natural electromagnetic EM fields as an energy source. The system includes the survey bird with electric dipoles, an angular motion detector and an airborne data recording system. The subject exploration survey system described herein is a substantial improvement over the survey system described in U.S. Pat. No. 6,765,383 and provides a unique airborne and/or ground survey system using a combination of remote sensing electric field EM techniques for oil, gas and mineral deposit exploration along with aquifer detection.